Ink jet printing is a non-impact method of printing that involves ejecting ink from a nozzle onto paper or other print media. The actual ink ejection method may occur via several processes including pressurized nozzles, electrostatic fields, piezoelectric elements within an ink nozzle, and heaters for vapor phase bubble formation.
The composition of the ink is traditionally comprised of deionized water, a water soluble organic solvent, and a colorant. The colorant may be a soluble dye or insoluble pigment. Several problems, however, are associated with soluble dyes that are not applicable to insoluble pigments. These problems include poor water-fastness, poor light-fastness, poor thermal stability, facile oxidation, dye crystallization, and ink bleeding and feathering on the print medium. To circumvent these problems, use of a pigment as the colorant is preferred. Pigments generally have better light-fast and water-fast properties, are more resistant to oxidation, and have higher thermal stability.
Use of a pigment instead of an aqueous dye presents solubility problems since the pigments are insoluble in aqueous media. As a result, the insoluble pigment is generally stabilized in a dispersion by a polymeric dispersant. Generally speaking most pigment inks stabilized by polymers in aqueous media are based on an electrosteric stabilizing mechanism in which a hydrophobic group in the dispersant acts as an anchor adsorbed on the pigment particle surface through an acid-base relation, electron donor/acceptor relation, Van der Waals forces, or physical absorption. The hydrophilic group is extended into the aqueous medium to keep the dispersant soluble. This results in a competition in the dispersing process between the pigment particle and the polymer, the polymer and the solvent, and the pigment particle and the solvent.
In order to form a stable polymeric dispersion, several factors are considered. First, the polymer must be firmly adsorbed to the pigment surface to withstand shear force and the competition of other chemical species. This requires a careful match of the polarity of the pigment surface to the hydrophobic group in the dispersant. Second, the physical dimensions of the hydrophobic group in the dispersant must be adequate to fully cover the pigment surface, otherwise, the adsorbed polymer will act as a flocculent. Third, an electrostatic layer of a requisite thickness around the particle is needed to repulse aggregation of particles within the aqueous medium.
In addition, many polymeric dispersions do not have acceptable smear resistance ability for some particular applications such as on photo paper or gelatin paper. The smear resistance of ink comprising a pigment dispersion can be improved in several ways, such as adding a latex binder or more penetrant to the ink formulation. But, in practice, penetrants reduce optical density and chroma, and binders may not work on photo paper coating. Therefore, rather than attempting to increase the smear resistance of pigment inks through the use of additives, the polymers of the present invention provide a dispersant that increases the smear resistance of pigmented inks, especially when used on photo or gelatin paper. The polymers of the present invention also provide excellent chroma for printing on photo paper.